This invention relates to an open gear lubricant and is particularly directed to open gear lubricants which are biodegradable.
Open gear lubricants are subject to particularly difficult operating conditions. Thus, not only must the lubricant perform its basic function of minimising friction and metal to metal contact between moving surfaces but it must also withstand the pressure, temperature and operating conditions found in difficult environments. Thus, for example, in mining operations, the machinery is exposed to an atmosphere of solid contaminants such as dust and minerals and to moisture in the form of humidity, rain and/or snow. Thus, the basic requirements for an open gear lubricant can be listed as follows
1. Tackiness and adhesion: the protecting film must strongly adhere to the surface to be lubricated without peeling or excessive throw-off; PA1 2. Extreme pressure resistance: should withstand heavy load and shock loading; PA1 3. Heat resistance: should not flow or harden in service and should not run even if applied on vertical surfaces; PA1 4. Water resistance: should withstand water washout; PA1 5. Mechanical shear stability: should not significantly change its consistency in service; PA1 6. Dust resistance: should be able to withstand incorporation of a large amount of dust without losing its lubricating properties; PA1 7. Pumpability: the product must be pumpable at low ambient temperature; PA1 8. Reversibility: should be stable under repeated hot and cold cycling. PA1 1. Mang T, Environmentally harmless lubricants, NLGI Spokesman, September 1993, Volume 57, Number 6. PA1 2. Dicken T. W., Biodegradable greases, Industrial lubrication and Technology, Vol 46, No. 3, 1994. PA1 3. Kitamura N, Biodegradable lubricants, Japanese Journal of Tribology, Vol 38, No. 5,1993. PA1 4. Honary L. a.t. (1994), Potential utilization of soybean oil as industrial hydraulic oil, SAE Technical Paper # 941760, Warrandle, Pa.: SAE Publications. PA1 5. Rohers I, Rossrucker T, Performance and ecology - two aspects for modern greases, Presented at the NLGI meeting, Palm Springs, Calif., Oct. 23-26, 1994.
Recent years have seen a growth in interest in the provision of environmentally friendly lubricants. This is particularly true for systems where the lubricants may be lost after use or accidentally come in contact with the environment. Several biodegradable precursors such as synthetic esters and vegetable oils have been proposed for use in lubricants. Thus, such lubricants are discussed in
However, the provision of a biodegradable lubricant, which is suitable for open gear applications with their severe performance requirements, has proved a difficult problem.
The present invention seeks to provide acceptable biodegradable open gear lubricants which perform at least as well as conventional mineral oil based products under a range of operating conditions.
According to this invention we provide a lubricant, intended for use in open gear applications, having a lubricant base which comprises a major amount of a vegetable oil and a minor amount of a solid inorganic lubricant, the lubricant being thickened with a biodegradable organoclay gellant.
The vegetable oil base stock can be selected from a wide range of available materials including canola, linseed, castor, sunflower, corn and soyabean oils which all exhibit high degrees of biodegradability whilst being abundant, renewable, economically viable and non toxic and exhibiting good lubricanting qualities in terms of lubricity, temperature-viscosity relationship (VI), stability and generally good seal compatibility. A preferred base stock is soyabean oil.
The vegetable oil is blended with a solid inorganic lubricant which is selected dependent on the intended conditions of operation of the lubricant. Thus, for some applications, such as mining equipment, walking cams, railroad wheels and switches, a suitable solid lubricant is a combination of carbon black and graphite while in other applications, especially where water resistance is required, a suitable inorganic lubricant is calcium carbonate. Other solid lubricants may be employed such as tricalcium phosphate and calcium hydroxide.
The preferred thickener is a biodegradable organoclay gellant such as Baragel 10 (an organoclay available from Rheox Inc.). Other clays may be employed such as montmorillonite and hectorite.
The lubricant may include a range of additional additives dependent on the end use and desired properties of the lubricant. These materials are selected so as not to adversely affect the global biodegradability of the product but to give better dispersability and stability, higher extreme pressure properties, better tackiness and adhesion and better resistance to water washout and inhibition to corrosion. Thus, for example, the lubricant may include minor amounts of additive selected from polar activators, plasticizers, anti-wear/ extreme pressure additives and metal deactivators.
The vegetable oil is preferably present in an amount of at least 50% by weight of the lubricant, more preferably 60-80%. The solid inorganic lubricant is preferably present in an amount of 15-30% by weight. The organoclay gellant is preferably present in an amount of 2-8% by weight.
The lubricant is preferably prepared by blending the organoclay gellant with the vegetable oil followed by incorporation of the inorganic lubricant and other performance additives. The additional performance additives are preferably added at a temperature which is kept suitably low enough to prevent decomposition. The organoclay gellant is preferably incorporated into the vegetable oil at high shear rates in order to delaminate the organoclay platelets for thickening the base stock. We have found that lubricants in accordance with the invention, while utilising the known useful properties of vegetable oils, overcome some of the perceived undesirable properties of vegetable oils such as low oxidative hydrolytic and thermal stability. Indeed, we have found that for open gear applications, the oxidising of the vegetable oil has desirable consequences in that a tough and strong lubricating film is obtained as a result of polymerization reactions, water resistance is substantially improved and thermal retention, tackiness and adhesion to the metallic surfaces are improved also.
The lubricants of the present invention are intended to be suitable for use in applications such as mining equipment, walking cams and railroad wheels and switches.